Field of Invention
The present invention relates to a physical vapor deposition (PVD) target. In particular, it relates to a target that is shaped like a truncated cone, a dome, or a Fresnel lens.
Discussion of Prior Art
Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by ejecting or “sputtering” material from a target onto a substrate, such as a silicon wafer.
Sputter coating apparatuses are generally known. In a typical apparatus, an energy discharge is used to excite atoms of an inert gas, e.g. argon, to form an ionized gas or plasma. Charged particles (electrons) from the plasma are accelerated toward the surface of a sputter target by application of a magnetic field. The sputter target typically is provided in the form of a rectangular slab, sheet, or plate. The plasma bombards the surface of the target, thus eroding that surface and liberating target material. The liberated target material then can be deposited onto a substrate, such as metal, plastic, glass, or a silicon wafer, to provide a thin-film coating of the target material on the substrate.
Sputtering sources can be magnetrons that utilize strong electric and magnetic fields to trap electrons close to the magnetron surface or target. These magnetic fields can be generated by an array of permanent magnets arranged behind the target, thus establishing a magnetic tunnel above the target surface. The electrons are forced to follow helical paths caused by the electric and magnetic fields and undergo more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. This results in a closed plasma loop during operation of the magnetron. At the location of the plasma loop on the surface of the target, a “racetrack” groove is formed, which is the area of preferred erosion of material. In order to increase material utilization, it is known in the prior art to use movable magnetic arrangements to sweep the plasma loop over larger areas of the target.
In order to decrease the racetrack groove formation and achieve more efficient utilization of the target, non-flat targets are known in the prior art. In general, it is a well known practice to increase the target thickness in the regions of main erosion. For example, U.S. Pat. No. 4,842,703 to Class et al. and U.S. Pat. No. 5,688,381 to Grünenfelder et al. disclose targets with a concave surface.
Several PVD applications require using a long distance between the target and the substrate. This is known as long target-to-substance distance (TSD) sputtering. Long TSD sputtering narrows the angular profile of the material sputtered from the target, making the sputtered material easier to direct. Long TSD sputtering is required in order to produce layers of film with low sidewall coverage, such as to enable lift-off processing or to avoid unwanted fencing of sidewall material when a photo resist is removed.
A disadvantage of a long target-to-substrate distance (TSD) is the resulting poor uniformity of the deposited material on the substrate. This effect can typically only be compensated by increasing the diameter of the target. However, increasing the diameter of the target can be burdensome and impractical. For example, if the substrate is a 300 mm wafer, a very big and uneconomic target size would be required.
Another disadvantage of long TSD sputtering is a dramatically reduced sputtering rate. In addition, because gas is scattering over an increased distance, the effect of narrowing the angular distribution is alleviated. In fact, the effect of directional sputtering almost disappears at realistic pressures of 1-2 mTorr, even at target-to-substrate distances as low as 150 mm.
FIGS. 1 to 5 illustrate the aforementioned issues with current sputtering targets. In particular, FIG. 1 illustrates a sputtering erosion profile of at a prior art target. In the graph of FIG. 1, the Y-variable is percent erosion of the target center area, and the X-variable is target radius at a point on the target in cm. The calculations are based on a target diameter of 400 mm and a substrate diameter of 300 mm at varying target substrate distances. The center of the target has a percent erosion of 10%. At a target radius between 10 and 15 cm, the percent erosion begins to increase until it eventually reaches 100%. This increased erosion represents the racetrack groove. FIG. 1 shows that while the material around the racetrack groove is completed eroded, plenty of useful material remains near the target center.
For the erosion profile of FIG. 1, FIG. 2 plots uniformity of the layer deposited on the substrate for target-to-substrate distances (TSDs) from 120 mm to 800 mm. The Y-variable is percent uniformity of the layer on the substrate, and the X-variable is substrate radius in mm. The graph of FIG. 2 shows that the flattest, and therefore most ideal, deposition profile is achieved for a TSD of 150 mm. For lower distances, such as 120 mm, the profile has a concave shape. For higher distances, such as 175 mm, 200 mm, 300 mm, 400 mm, or 500 mm, the profile gets convex. However for very long distances, such as 800 mm, the convexity decreases since the target can be seen more and more as a point source.
In FIG. 3, the behavior of FIG. 2 is plotted as function of TSD. The Y-variable is percent surface fluctuation of the deposited layer on the substrate, and the X-variable is TSD in mm. Similar to FIG. 2, FIG. 3 indicates the best uniformity around a TSD of 150 mm.
FIG. 4 depicts sputtering efficiency based on the same conditions as FIGS. 1-3. In FIG. 4, the Y-variable is percent deposition efficiency, and the X-variable is TSD in mm. As the TSD increases, the deposition efficiency decreases.
It should be appreciated that these calculations have been done for very low pressures of 0.1 mTorr. When the pressure is increased, which may be necessary to sustain stable plasma, the uniformity gets much worse. FIG. 5 depicts this pressure effect on uniformity for a TSD of 500 mm. The Y-variable is percent surface fluctuation of the deposited layer on the substrate, and the X-variable is substrate radius in dm.
Thus, there is need for improvements in sputtering targets in order to more evenly erode the target while having positive uniformity and efficiency characteristics.